So Timo, based on your "If they can't make it in this universe, they should be let to die," statement, are you one of the folks who believes the Prime Directive shouldn't be violated to save a species endangered by, say, an asteroid strike?
I'm canon-powered at heart, so for TOS I must believe that the PD would not come in the way of such salvation ("Paradise Syndrome"), but for TNG I must believe the opposite ("Homeward"). Yet altruism never seemed to power Kirk: he remorselessly eradicated some species in order to make life better for others, chiefly because these others either were like him, or had the potential to become like him. So some sort of vanity would seem to be the primary motivator here, and that should be compatible with letting die any creature or species that did not directly contribute to the welfare of Kirklings.
The haemoplasm was in a sealed jar, which the creature penetrated with ease.
But our heroes would have had every motivation to prepare the bait so that it would be easily penetrated. In contrast, a sealed starship proved impenetrable, so that the creature had to channel itself through an actual opening - which led it, to its detriment, to a cabin occupied by a Vulcan. It probably mistook the entire innards of the ship for inedibles because of that... Remarkably, it did not even penetrate the apparently less than airtight doors of the cabin (see the crack below?) to seek for tastier stuff. So even "faking" a barrier should be of help.
How does the beast choose an edible target? If it failed to realize there were edibles inside the starship, just a few feet away, it probably needs to
smell the prey. So a "penetrable" spacesuit (or even a TAS style life support forcefield) should again suffice for protection. That's how you survive in the battlefield, too: flak jackets don't really help against bullets, but camouflage uniforms do!
You forget how fast it can move. If it can travel faster than a starship, it can threaten any world of UFP interest.
It can, but it didn't. For eleven years it hid. For untold years before that, it hid. It would be absolutely crucial to know why this was, and where it hid, and why, when and how it decided that it was time to feed on humanoid blood.
A lifeform can be rare due to circumstances beyond its control. Yet, you still think it shouldn't be preserved, even if we could learn a great deal from it?
Depends on the effort required. Our heroes never engaged in any effort that would put human(oid)s at a disadvantage; any preservation cheaper than that would probably indeed be done, out of sheer scientific curiosity.
And um... if we DO kill one then we know how to kill another, btw!
With the Borg, that's the exact opposite of the truth.
Intriguingly, the Vampire Cloud appeared to be a master of adaptation, too - fortunately, we saw no signs of a collective mind that would let other members of the species learn from the death of one. But if we kill our potential Judas goat, then the herd cannot be killed. At the conclusion of "Obsession", our heroes had no idea how or where or when to find further Vampire Clouds. That's the crucial step in killing them: once located, they can apparently be lured to become opaque to antimatter blasts for the duration of feeding on a blood bait. And a blast somewhat smaller than the impracticable one used in the episode might well be devised, once our heroes learned more about the creature. (Except now they can't.

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Timo Saloniemi